Chris Peterson wrote:
[...] Not everything is a quantum effect. But diffraction is. Diffraction is only *fully *understood by utilizing QM. Classical theory is only an approximation. It's a very good approximation in many cases (but not so good in the case of diffraction by small particles), but an approximation all the same. [...]

I have no complaint with the caption describing this effect as the product of quantum mechanical diffraction, only with the use of the word "purely", which is somewhat confusing.

Yes, the interaction with light and matter is, when you get down to it, of quantum mechanical nature. But I don't understand why you need to employ QM to describe diffraction. You need QM if you want to calculate material properties that are important to diffraction, such as the polarisability of the atoms. Once you take these properties as given, you can employ classical scattering theory.

Also,

*classical* Rayleigh scattering applies when the scatterer is much smaller than the wavelength in question. This is why it successfully describes the blue sky, etc, where the wavelength of visible light is a few hundert nanometers compared to the size of the air molecules (~0.1 nm). Only when the wavelength becomes comparable to the atom radius (e.g. x-rays) do you need QM.

The works by Rayleigh were published in the 1870, well before QM, and those of Mie around 1910 (most likely still classical, but I didn't look up the references).

Chris Peterson wrote:All diffraction is a quantum effect. That is, while it can be approximately described by classical theory, in certain regimes it is best explained using QM. A rigorous analysis of Mie scattering and the color effects seen in a lunar corona is best handled with QM. I think it is slightly misleading to say that this effect is "purely" quantum mechanical, however.

I looked up

Mie scattering on Wikipedia and in

Hecht. In Mie scattering, the cross section is independent of the wavelength, while today's APOD shows an effect which is wavelength dependent. My guess is that it is related to Rayleigh scattering (which, of course, is a limiting case of Mie scattering). Do you know a reference that discusses the lunar corona with QM?